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Over the last 10 years, advancements in solid modelingtechnologies and subsequent changes in the injection

molding industry have created the need for more complex anddiverse assembly solutions. Due to the more complex andstringent requirements for strength, tightness, and aestheticcharacteristics in the electronic, medical, and automotive com-ponent assembly, the need for a non-violent or degrading methodfor plastic assembly has increased. There are a multitude oftechnologies and companies that produce machines for plasticassembly. A few of these plastic assembly technologies in-clude infrared, laser, hot air/cold upset, and thermal impulse,as well as the welding technologies - ultrasonic, vibration, hotplate, RF, and spin. Many of these technologies have limita-tions that can dramatically affect the assembly process suchas material selection, filler material (additives used), cycle time,tooling costs, and particulate generation (debris related to thevolatile nature of the assembly process used).

Assembly challenges such as attaching components topopulated printed circuit boards or complex plastic parts canbe overcome by using the heat staking assembly process.The process of applying heat and pressure to stake, swage,or seal the post(s), tab(s), wall(s) or an area of one or moreplastic parts to other part(s) which could be pcb(s), metalframe(s), or other plastic part(s), is commonly referred to asheat staking. There are several specific processes withinthe heat staking family. The accurate description for theheat staking process is using precise heat and pressure toreform a post or boss molded into a plastic part to retain orattach the plastic part to another component or part, such asa pcb, metal frame, etc. Heat swaging is a similar process toheat staking, but it involves rolling or reforming a wall (typi-cally a perimeter) of a plastic part to retain another part orcomponent. Heat insertion typically refers to the retainingof a threaded insert into a molded hollow boss of a plasticpart. Heat sealing is another process that uses heat andpressure to seal or adhere an area of one plastic part or filmto another part(s). Heat sealing is also an easy and consis-tent process used to apply heat seal connectors to PCBs orLCDs in many consumer electronic devices.

Not only are heat staking, swaging, and sealing the pre-ferred methods for consumer electronic devices, but theyalso are the assembly method of choice for the followingapplications: multi-level staking or insertion projects; stakingmultiple threaded inserts at one time; applying heat seal con-nectors to an LCD/PCB; and swaging around the perimeterof a part for retention purposes. Damage to componentsand cold solder joints on PCBs caused by ultrasonic welding

make heat staking the only viable staking solution for attach-ing a PCB into a plastic housing. Heat staking typically outperforms its competition in cost, repeatability, strength, qual-ity, reliability, and even throughput.

The heat staking process typically requires the tempera-ture to be high enough to reset the plastics� memory, but notso high as to melt the plastic. Most thermoplastics have afairly wide operating temperature window when assembledwith a heat staking method. Higher pull strengths are typi-cally achieved when the heat staking process is used forstaking, swaging, and insertion applications.

Most processes that require melting of the plastic duringthe assembly process often create particulate or debris. Thisdebris is an unacceptable side effect in most medical andconsumer electronics applications. Although a lower tem-perature is used during the heat staking process, it is offsetby the increased amount of force required to deflect thematerial without damaging the plastic.

With the heat staking method of assembly, there is an in-verse relationship between the temperature and the forcerequired to achieve an optimum completed stake, swage, orinsertion. When optimizing the parameters for a specificplastic substrate, if the temperature is lowered then the forcerequired must increase. To overcome the rebound of somethermoplastics or part design characteristics, optional postcooling technologies can be implemented.

Post cooling is the process of applying cold air towardsthe tip(s) of the thermal probe(s) or thermal tooling duringthe end of the staking, swaging, or insertion process. Thislowers the temperature of the tooling and the plastic, whilestill held under the pressure of the thermal tool(s), whichnow act as a clamping mechanism.

The heat staking process has very basic design standardsthat are recognized throughout the plastic assembly industry.When these design standards are followed the heat stakingprocess produces strong, consistent, aesthetically pleasingassembled parts.The heat stakingpost design stan-dards are describedas follows:

The height of thepost, or portion to bestaked, should be 1 - 2-1/2 times the diameter of the postabove the surface. Typically, 1-1/2 times the diameter of thepost will work for most applications. Post head geometrycan vary, depending on the application requirements.

By Gary Sullivan and Lance Crawford - SAS and Thermal Press Int'l, Inc.

Plastics AssemblyPlastics Assembly

The Heat Stake AdvantageThe Heat Stake Advantage

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Posts can be round, square, cross-shaped, or hollowbosses. Different post designs can be selected for specificcharacteristics or aesthetic requirements. Round and squareposts are the most commonly used, due to the ease in designand implementation into an existing or new mold cavity. Cross-shaped posts are typically used when design constraints dueto wall thickness, overall post height, or possible sink markson the opposing side of the part dictate the need for an alter-nate design concept.

Most heat stake posts can be staked adequately with aflat tipped thermal probe. Sometimes when higher breakstrengths or better aesthetics are required, a dome shapedprofile can be cut into the thermal probe. The dome shapedprofile will have to be accurately engineered, so that the mini-mum volume of the plas-tic to be staked can fit in-side of the dome-shapedprofile heat-staking tip. Ifthe dome-shaped designis done correctly, no flashwill be created aroundthe perimeter of the fin-ished heat stakes. Dome-shaped heat stakes require the greatest amount of force com-pared to the other standard staking profiles.

Hollow boss posts are typically used when high retentionis required. Hollow bosses are also used for larger heatstaking post applications. Typically, hollow bosses are notsmaller than 3/32� in diameter. A hollow boss post heat stakedwith a rosette shaped profile can create a substantially tighterand stronger heat staked assembly, while also producing one

of the most aestheticallypleasing finishes. A ro-sette profile is a curvedhollow ring, similar inshape to a rivet. A ro-sette heat stake gets itsstrength from how the

profile in the tip rolls a large amount of material outboundaround the perimeter of the hollow boss, creating a largecontact surface area. The alignment for the thermal probesperforming the rosette heat staking process requires greateraccuracy than with flat tipped thermal probes.

A knurled profile on the staking tip is used in high volume op-erations where accuracy and aesthetics are not limiting factors.A knurled tip or surface can produce a tight bond very quickly.

Heat staking can be performed inside of counterbores andcountersinks to keep the material below the surface, if needed.Heat staking into counterbores is relatively straight forwardusing a flat or dome-shaped staking probe, while heat stak-ing into countersinks requires that the design be accurate, sothe volume of the staking post will fill the countersink cavity.

The heat staking ofthreaded inserts requires adifferent style thermalprobe(s) than that whichis used for a standard heatstaking application. Typi-cally, temperatures arehigher for insertion appli-cations due to the thermalmigration through the in-sert prior to contacting the plastic part.

De-gating or un-staking of components using the heat stak-ing process is not only fast, but can be done with very littleparticulate, a problem typically found with ultrasonics. Aswith staking, melt temperatures are not required. De-gatingprobes are similar to staking probes, but have a pointed tip tohelp move the existing heat stakes away. !

Gary Sullivan was the National Sales Manager for Spe-cialized Assembly Systems, LLC. Lance Crawford is theVice President of Sales for Thermal Press International, Inc. For more information on plastic assembly tecnologies, techniques, and equipment, please contact Lance Crawford

at (925) 454-9800 or by email: lcrawford@thermalpress.com.